Projection exposure apparatuses for microlithography, hereinafter called projection exposure apparatuses, generally include: a light source; an illumination system, which processes the light rays emitted by the light source to form illumination light; an object to be projected, generally called a reticle or a mask; a projection objective, hereinafter called an objective, which images an object field onto an image field; and a further object onto which projection is effected, generally called a wafer. The mask or at least part of the mask is situated in the object field, and the wafer or at least part of the wafer is situated in the image field.
If the mask is situated completely in the region of the object field, and the wafer is exposed without a relative movement of wafer and image field, then the projection exposure apparatus is generally referred to as wafer stepper. If only part of the mask is situated in the region of the object field, and the wafer is exposed during a relative movement of wafer and image field, then the projection exposure apparatus is generally referred to as wafer scanner. The spatial dimension defined by the relative movement of reticle and wafer is generally referred to as scanning direction.
During the exposure of the wafer, the mask is illuminated with illumination light by the illumination system. The type of illumination is designated as a setting. A distinction is made between coherent illumination, incoherent illumination with a a setting of between 0 and 1, annular illuminations, X- or Y-dipole settings with different, illuminated aperture angles, and quadrupole settings. Current development is tending in the direction of freeform illuminations, such as described, for example, in “Illumination Optics for Source-Mask Optimization, Yasushi Mizuno et al., Proc. SPIE 7640, 764011 (2010)”. In this case, the intensity of the illumination light in the exit pupil of the illumination system can be set in any desired manner with a high spatial resolution.
For the integration density of the integrated circuit to be produced with the aid of the microlithographic exposure process, periodic structures can be very important. The structures are described by pitch and structure width. The structure width used on the wafer can be set freely to a certain degree by the resist threshold of the resist to be exposed. By contrast, the smallest achievable pitch, Pitchmin is given by the wavelength of the illumination light and the object-side numerical aperture of the objective. The following holds true:
      Pitch    min    =      λ          NA      ⁡              (                  1          +          σ                )            for coherent and incoherent illumination with a predefined σ setting σ.
The structures on the mask to be imaged generally have two preferred directions. In the assessment of the imaging qualities of a projection exposure apparatus, therefore, a distinction is made at least between the maximally resolvable pitch of H (horizontal) and V (vertical) structures. In this case, hereinafter an H structure means a sequence of light-transmissive and light-opaque regions of the mask, wherein each individual one of the regions has its larger extent orthogonally with respect to the scanning direction.
The integration density ultimately achievable on the wafer in a projection exposure apparatus is substantially dependent on the following parameters: (a) depth of focus DOF of the objective, (b) image-side numerical aperture NA and (c) wavelength λ of the illumination light. Reliable operation of a projection exposure apparatus involves, for a desired critical dimension CD (that is to say the smallest structure width occurring on the wafer) and a given numerical aperture NA, the largest possible so-called process window formed from possible defocusing FV (focus variation) and variation of the dose of the illumination light. In this case, NA and DOF are anti-proportional. In order to reduce the critical dimension CD further, development is generally tending toward increasing numerical apertures NA. However, this leads to a reduction of the depth of focus DOF and thus to a reduction of the process window.
Therefore, it is desirable to increase or at least stabilize the process window in the context of a decreasing critical dimension CD.
Currently good resolutions of pitch and CD are achieved by two classes of projection exposure apparatuses.
The first class of projection exposure apparatuses is operated with an ArF laser at a wavelength λ of the illumination light of 193 nm with polarized light and works in immersion operation, that is to say with a liquid as last medium before the wafer, or in dry operation, that is to say with a gas as last medium before the wafer. The associated objectives that image the illuminated mask onto the wafer are generally dioptric or catadioptric objectives. The latter are operated with image-side numerical apertures of 0.8 or 1.3 or higher. By way of example, cf. US 20060139611A1, US 20090034061A1 or US 20080151365A1. The reticle is generally a glass substrate, and the structures of the reticle are defined by a structured layer composed of Cr, MoSi or other materials on the substrate.
The second class of projection exposure apparatuses is operated with a source of weak X-ray radiation (commonly referred to as EUV, extreme ultraviolet) at a wavelength λ of the illumination light of 13.5 nm. Such apparatuses are commonly referred to as EUV systems or EUV projection exposure apparatuses. The associated objectives that image the illuminated mask onto the wafer are catoptric objectives. The latter are operated with image-side numerical apertures of 0.2 to 0.35, 0.9 or higher, such as described, for example, in US 20050088760A1 or US 200801700310A1. The reticle is generally a glass substrate, such as ULE™ or Zerodur®, which becomes highly reflective through a stack of alternating Mo and Si layers in the case of light having a wavelength λ of 13.5 nm and the structures of the reticle are in turn defined by a structured Cr layer or else by a structured layer composed of TaN or other materials. The thickness of the structured layer is typically 50-70 nm.
The effects explained in “Polarization-induced astigmatism caused by topographic masks, Ruoff et al, Proc SPIE 6730, 67301T (2007)” occur in the first class of projection exposure apparatuses. Accordingly, the TM- and TE-polarized components of the illumination light respectively lead to different positions of the foci of H and V structures. Wavefront aberrations and in this case specifically astigmatism Z5, Z6 accordingly occur in the case of polarized illumination of a mask and subsequent imaging by the objective. The astigmatic terms Z5, Z6 are Zernike polynomials, the indexing of which follows fringe notation; cf. “Handbook of Optical Systems, Singer et al (eds.), Wiley-Vch, 2005”. This is substantially associated with the failure of the Kirchhoff approximation upon the diffraction of the illumination light at the mask, which then appears three-dimensional for the light, in conjunction with a polarized illumination. The abovementioned article specifies a generalized Kirchhoff approximation that takes account of these effects. These effects, called rigorous in the technical jargon, are dependent on the structure widths, the material that defines the structures of the mask, such as Cr, for example, and the thickness of the structures in the direction of the beam path of the illumination light in the region of the mask.
Therefore, in this first class of projection exposure apparatuses, it is desirable to correct the structure- and pitch-dependent aberrations of the wavefront which are cause by the rigorous effects of the mask, and in particular, it is desirable to correct the structure- and pitch-dependent astigmatism induced by the mask.
In this case, the wavefront aberration induced by the structure or the structure width or the pitch or the induced aberration of the wavefront is understood hereinafter to mean the aberration that is caused exclusively by this structuring of the mask. To put it another way, this is the aberration that arises in addition to the other aberrations of the objective that are already present. Instead of an induced wavefront aberration of a pitch or of a structure width, reference shall also be made just to the wavefront aberration of the pitch or of the structure width.
In the second class of projection exposure apparatuses, the illumination of the mask takes place in reflection. Therefore, no telecentric illumination of the mask is possible since otherwise the illumination system and the objective would be in the way. The chief ray angle CRA, in the case of a projection exposure apparatuses in the first class, is the deviation of the chief ray from a telecentric ray. In the present case of a projection exposure apparatus in the second class, it is the angle of the chief ray of the illumination light with respect to an imaginary orthogonal relative to the object plane of the objective. In the case of a projection exposure apparatus as presented in US 20050088760A1, a CRA of 6° is used in the case of an image-side numerical aperture NA of 0.33. In the case of a projection exposure apparatus as presented in US 200801700310A1, a CRA of 15° in the case of an image-side numerical aperture NA of 0.5 is used. Generally, the CRA used increases with the numerical aperture NA of the objective.
For the effects described below, also cf. with “Mask diffraction analysis and optimization for EUV masks, Erdmann et al, Proceedings of the SPIE—The International Society for Optical Engineering 2009, vol. 7271”.
As illustrated in a more detailed manner below, the CRA different from 0° results in shading of the reflected illumination light by the extent of the mask structures orthogonally with respect to the object plane of the objective. Therefore, here a purely topographical effect of the mask is present, which is determined by the geometrical, three-dimensional arrangement of illumination system, mask and objective. However, in contrast to the first class of projection exposure apparatuses, the effect also influences undiffracted illumination light.
This effect can no longer be disregarded, precisely for EUV projection exposure apparatuses, since the thickness of the structured layer on the mask is several wavelengths of the illumination light of λ=13.5 nm, and thus, in contrast to the first class of projection exposure apparatuses, which is operated at a wavelength of λ=193 nm, reference can be made to shadow casting.
If only undiffracted illumination light is considered, then this shading is manifested to a greater extent for H structures than for V structures if, for the design of illumination system and objective, it is assumed that the plane of incidence of the CRA on the mask is orthogonal to the extent of an individual structure of the H structures. The magnitude of the difference between the structure widths of H and V structures on the wafer, assuming identical structure widths on the mask, is dependent on the position thereof as an object point, considered in the object plane of the objective, as will be illustrated later. Therefore, H structures are generally imaged wider, depending on their position in the object plane of the objective. In addition, an image offset dependent on the position of the object point arises for H structures, which corresponds to a field-dependent tilt of the wavefront Z2, Z3. If the entire wavefront is analyzed, then the field-dependent distortion terms Z2, Z3, defocusing Z4 and astigmatism Z5, Z6 arise as aberrations. These are accompanied by higher-order wavefront aberrations such as coma Z7, Z8 and secondary astigmatism Z12, Z13.
Therefore, in this second class of projection exposure apparatuses, too, it is desirable to correct the structure-width- and pitch-dependent aberrations of the wavefront which are induced by the rigorous effects of the mask, and, in particular it is desirable to correct the structure-width- and pitch-dependent astigmatism induced by the mask, of structure-width- and pitch-dependent distortion that is dependent on the position of the object point, and also of structure-width- and pitch-dependent focal position.